Various processes require a textured or topographically patterned solid surface where chemical composition and/or physical properties, such as catalytic activity or wetting characteristics, differ on recessed and non-recessed areas of the surface. For example, variations in the wetting characteristics on recessed and non-recessed areas of printing plates are exploited to selectively accept and/or transfer ink. Such variations can be achieved by selectively contacting or coating the non-recessed areas with surfactants, polymers or chemical reagents such as oxidizing agents. However, when the difference in elevation of recessed and non-recessed areas is small, for example less than 1 micron, mechanical methods for applying or removing coatings, surfactants or reagents are often impractical or insufficiently selective.
Interconnections on integrated circuits are fabricated by the Cu damascene process in which the interconnect circuit pattern is lithographically etched into the surface of a dielectric layer on the surface of the wafer. The etching creates “recessed areas” in the dielectric layer that can be several nanometers to several microns in depth. The remaining surface of the dielectric layer forms “non-recessed areas” that surround the “recessed areas.” The pattern is then coated with thin conformal layers of a barrier metal, such as Ta, followed by Cu. Additional Cu is then electroplated over the entire surface of a wafer to fill completely the recessed areas with Cu.
In order to assure complete filling of the smallest recessed areas, the electroplating chemistry incorporates an electrocatalyst, most commonly 3-mercapto-1-propane sulfonic acid (MPS), a salt of MPS, or a corresponding disulfide. These electrocatalysts adsorb to the Cu surface and increase the rate of Cu electrodeposition relative to areas of bare Cu lacking an adsorbed electrocatalyst. This effect is amplified in very small recessed areas because the surface concentration of the electrocatalyst increases during filling. Because the electrocatalyst is present on all surfaces of the wafer, in the course of filling larger recessed areas, excess Cu is deposited everywhere and must be removed.
Chemical mechanical polishing (CMP) can be used to remove the MPS from the non-recessed areas before ECD, but it also tends to remove some of the thin layer of Cu on the non-recessed areas and can lead to defects in the final product.
Alternatively, the MPS-coated wafer can be exposed to strong oxidizing reagents such as K2FeO4 or H2O2 in bulk solution before ECD to convert the strongly adsorbing thiol group of MPS to more weakly adsorbing sulfinic or sulfonic acid groups. Although this method may not affect the thin layer of Cu, the extent of oxidation and removal of MPS from the recessed and non-recessed areas is nearly equal, resulting in little or no difference in the rates of subsequent ECD.
Ozone is a strong oxidizing agent that can be readily generated in the gas phase from oxygen by electrical discharge. When dissolved in water, ozone decomposes rapidly to a mixture of short-lived intermediates, including superoxide (O2−), hydroxyl (HO) and peroxy (HO2) radicals, and is useful as a biological disinfectant and for removing photoresist polymers from silicon wafers and liquid crystal displays. Neither ozone nor aqueous ozone has been used in a commercial process to selectively remove absorbed organic materials from non-recessed areas of a topographically patterned surface.
A method is needed that removes electrocatalysts from the non-recessed areas of a surface containing recessed and non-recessed areas, without removing electrocatalyst from the recessed areas and which removes little or no Cu from the surface. More generally, a method is needed for selectively removing organic compounds coated or adsorbed on the non-recessed portions of a topographically patterned solid surface.